Radiographic images of anatomical regions are a routine and valuable diagnostic and research tool. Such images are typically produced by placing an object, such as a portion of the human body, on a plate having a surface composed of excitable phosphors (a film) and exposing the object and film to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, ultraviolet rays, or the like. As the radiation energy strikes the surface of the film, a portion of the energy is stored by the phosphor-containing film. Upon subsequent stimulation by visible light or other stimuli, the film gives off light in direct proportion to the amount of energy stored therein. Areas of the film receiving unattentuated radiation absorb the most energy and thus produce the most light when subsequently stimulated. Areas in which lesser amounts of radiation energy are absorbed, due to the presence of the object (e.g., body region), produce a proportionately lesser amount light when subsequently stimulated.
The image is displayed for viewing in one of several ways. For example, the stored energy of the film can be photoelectrically detected and converted into a signal which is then further processed or used to reproduce the image on a photographic film, CRT or similar device.
One of the most common radiographic images utilized in clinical settings today is an image of the thoracic area of human body (e.g., chest x-ray). Such images provide invaluable information and are used to diagnose maladies ranging from lung and breast cancer to emphysema.
A radiographic image of an anatomical region like a chest x-ray contains three main regions: (1) the foreground; (2) the background; and (3) the anatomical region of interest. For purposes of this Application, the term "background" is used to denote the very high intensity regions of a film, wherein unattenuated radiation energy was absorbed by the film (i.e., the area in which no body portion or object was present). "Foreground" will be used herein to designate the very low intensity regions of the film, wherein highly absorbent structures (e.g., collimator blades) are used to "frame" the field of irradiation on the film. The anatomical region of interest for purposes of illustration only will be the chest and thoracic area of the human body.
The quality of information obtainable from a digital radiographic image, such as a chest film, depends on several factors, including the device used to acquire the original image, any image enhancements and/or analysis performed on the original image, the capabilities of the output device, and the positional orientation of the anatomical region of interest within the image.
The positional orientation of the anatomical region of interest with respect to the film is of perhaps obvious critical importance to the image interpreter (e.g., radiologist), as well as for further processing of the image.
Today most radiologists interpret films from light boxes. When the films are brought to the light boxes and placed by technicians, secretaries, or other personnel, the films are expected to have a correct orientation to facilitate proper reading. Correct orientation takes on increased importance where radiologists compare the newly acquired film with one or more previous films of the anatomical region of interest.
Additionally, proper positional orientation of an image is necessary for automated image enhancing and reading systems, which often assume an arbitrary positional orientation for any given image signal. For example, where an image will be viewed on a CRT or similar device the positional orientation is assumed to be parallel with the longitudinal direction of the rectangular phosphor surface on which the image was created (i.e., the neck portion of the image is aligned with the "top" of the film). Accordingly, feeding an image signal oriented in any position other than parallel with the longitudinal direction of the phosphor surface produces errors in the display and prevents accurate further enhancement and/or processing of the image.
Due to the importance of this information, prior art attempts at detecting positional orientation of a radiographic image are known. For example, one prior art method compares the sum of intensity values of the image signal of a horizontal strip of the subject film with the sum of the intensity values of the image signal of a vertical strip of the subject film. The strips are taken from the subject film without differentiating the source of the intensity values within the strip (i.e., there is no way to tell if the intensity values are from the body region of interest, the background or the foreground). Accordingly, if a portion of the foreground (very low intensity values) or background (very high intensity values) of the film is included in the strip, the calculations of distribution are skewed in one or both directions. In such instances, the ultimate determination of orientation lacks consistency and thus reliability.
Another prior art method utilizes a comparison of the characteristic values (sum of the intensity values and the average intensity value) for a horizontal strip of the film and for a vertical strip of the film. Although this method purportedly ensures that the strips will pass through the central portion of the image, this method also lacks any differentiation of the source of the image values used in the calculations. Thus, it is also impossible with this prior art method to determine whether, and by how much, the results are skewed by the inclusion of a portion of the background and/or foreground of the film. An additional problem associated with this prior art method is that the method assumes perfect alignment of the body region of interest on the film to ensure that the strips are taken through the central portion of the image. This assumption ignores the reality of actual image production.
A different prior art method attempts to determine whether a digital chest radiographic image has line symmetry or not, based on the thoracic vertebrae extending vertically in the image signal. Although this method also may be used to detect the orientation of a digital chest radiographic image, the problem of lack of differentiation of body region of interest versus foreground and background intensity values also exists in this method. Additionally, once again, the method makes the unrealistic assumption that the body region of interest is perfectly centered in the image signal.
It is observed that methods without distinguishing between the desired regions of the image (the body region of interest) and the undesired regions of the image (the foreground and background) are suboptimal, producing inconsistent and unreliable results.
Thus a need remains for an automated method and system for digital image processing of radiographic images to determine positional orientation wherein the undesired portions of the image (the foreground and background) are distinguished from the desired portion (the body region(s) of interest) and are excluded from the analysis.